1. Field of the Invention
The invention relates to a light source device using a high pressure mercury discharge lamp which is used, for example, as a light source for a projector.
2. Description of the Prior Art
In a light source device for an optical device, such as a liquid crystal projector, a DLP or the like, a discharge lamp with high radiance (HID lamp) is used. Recently, high pressure mercury discharge lamps have been used more and more due to their high efficiency of conversion of their making power into light intensity.
For driving a discharge lamp there are a direct current (dc) drive and an alternating current (ac) drive. In the dc drive the polarity of the two poles of the electrodes of the discharge lamp, i.e., the condition that one of the electrodes is a cathode and the other electrode is an anode, is not changed. In the ac drive the relation between the anode and cathode is changed essentially periodically. In the ac drive, with reference to the speed of the polarity change, i.e., with reference to the control frequency, driving in a wide frequency range from a few dozen hertz to a few megahertz is enabled.
With respect to the arrangement of the feed device for supplying the discharge current to the above described discharge lamp, expressed in very rough terms, the following is described.
In a feed device for the ac drive, downstream of a ballast circuit, such as a chopper circuit or the like, and in a feed device for a dc drive, an inverter for inverting the polarity is added.
Since operation of the feed device and the behavior of the discharge lamp in dc drive and ac drive are the same when this discharge lamp is started, if we consider that the above described inverter is excluded, mainly a case of dc drive is easily described below using FIGS. 16(a) and 16(b). In FIG. 16(a) the time change of the lamp voltage (VL) is shown schematically, while in FIG. 16(b) the time change of the lamp current (IL) is shown schematically.
The starting sequence of the feed device is initiated with the application of a voltage, which is called the no-load voltage, to the discharge lamp. An interval (T01) corresponds to the start of the starting sequence. At an instant (tz) a starter is operated which produces a high voltage. Between the two poles of the above described discharge lamp, a high voltage (Vz) is applied and in the discharge space an insulation breakdown is produced, the latter acting as a cause for the discharge being started.
Following this insulation breakdown, a brief reduction (Vza) of the lamp voltage (VL) becomes apparent. But depending on the adhesion state of the mercury to the lamp there is also a case in which this cannot be clearly observed, or a case in which it can be detected for a few milliseconds or over a longer interval. Following the insulation breakdown a relatively large equalization current (Iza) flows in the lamp. This is because, as a result of the acute reduction of the lamp voltage (VL), the electrical charge stored in a smoothing capacitor of the ballast circuit is released.
After a relatively unstable discharge, directly after one such insulation breakdown, a transition occurs to a discharge which is called a glow discharge.
Generally a glow discharge has a higher discharge voltage and a lower current density than an arc discharge. When in a stable glow discharge interval (T11) the temperature of the electrodes, especially of the cathode, increases enough and a temperature is reached at which thermionic emission is possible, the discharge form passes into an arc discharge and a transition to an initial interval (T31) of an arc discharge takes place, in which the lamp temperature is not yet high enough, in which therefore the vapor pressure of the mercury is low, and in which therefore the discharge voltage of the lamp is low. In this initial interval (T31) of the arc discharge, normally for the feed device of the lamp the nominal electric power cannot be supplied. This is because an overly large current must be allowed to flow when the attempt is made to reach the nominal electric power at an overly low voltage, and because overly high heat generation of the switching devices of the feed device takes place, the electric power being computed as the product of the voltage and current.
In an interval (T32) in which the lamp temperature slowly rises, the lamp voltage increases over time. When the lamp current drops en route, which is necessary to achieve the nominal electric power, and when it drops to less than or equal to an upper boundary value ILmax of an allowable lamp current of the feed device, the nominal electric power is supplied to the lamp until a steady state (T33) is reached and maintained. Normally, this takes roughly 1 to 2 minutes.
With respect to the lamp service life, there are several different factors, such as for example wear of the electrodes which depends on the cumulative length of illumination of the lamp, the accumulated stress on the bulb glass which depends on the thermal behavior, and which causes lamp damage, and similar factors. There was especially the disadvantage of formation of blackening on the inside of the bulb glass as a service life factor in conjunction with a glow discharge.
As was described above, a glow discharge has a high voltage. The cations in the discharge plasmas are accelerated by high energy and collide with the cathode. As a result, the cathode material, such as tungsten or the like, vaporizes by sputtering. It can be imagined that on the inside of the above described bulb glass blackening therefore occurs and that the above described vaporized material is deposited on the inside of the bulb glass. Due to this factor, the lamp service life therefore progresses depending on the frequency of flashing and luminous operation of the lamp.
Such suitable control of the behavior of the system in a glow discharge, i.e., during the stable glow discharge interval (T11), for an interval (T12) during which the glow discharge increases, and during an interval (T21) during which the glow discharge drops, as shown in FIGS. 16(a) and 16(b), is a very important element for the lamp service life. Nevertheless, until now, a sufficiently attentive check has not been done; this was especially the case in a HID lamp which contains greater than or equal to 0.15 mg mercury per cubic millimeter of volume of the discharge space.
Japanese patent disclosure document 2001-6895 (U.S. patent application Ser. No. 09/720,092; EP 1139700 A1) discloses an invention in which a feed device for a high pressure mercury discharge lamp which contains greater than or equal to 0.15 mg mercury per cubic millimeter volume of the discharge space is arranged such that for a glow discharge an electric power is allowed which exceeds a certain value.
However, here, a temporary character feature is mentioned which a feed device is supposed to have which can prevent the discharge from disappearing when the discharge state of the discharge lamp returns from the initial interval (T31) of the arc discharge to the glow discharge after a transition to the arc discharge has taken place once.
This means that the description of the invention and the prior art in the aforementioned publication does relate to the temporary behavior described below in movement from one point to another point on the characteristic (Fp0) of the constant electric power and on a characteristic which relates to the area between a point (P0g) and a point (POc) in the invention described in these application documents in FIG. 2. The specification of the invention and description of the prior art in the above described publication however did not mention at all the disadvantage of formation of blackening on the inside of the bulb glass for the duration of the glow discharge in itself as a result of an output current-voltage characteristic in the glow discharge according to the characteristic shown in FIG. 2 (F1a).
Recently, there has been a demand to save energy to prevent global warming. With respect to the above described optical device, it is therefore desirable to turn it off as often as possible. But since the above described service life, as a result of blackening in a glow discharge, progresses depending on the frequency of flashing and luminous operation of the lamp, with respect to the lamp service life a mode of use is advantageous in which there is as little flashing as possible. One such mode of use however contradicts the above described demand for saving energy.
The object of the invention is to eliminate the disadvantage in the prior art, i.e., the disadvantage of the formation of blackening on the inside of the bulb glass in a glow discharge.
This object is achieved in accordance with the invention as follows.
In accordance with a first aspect of the invention, the object is achieved in a light source device in which a discharge lamp which contains greater than or equal to 0.15 mg mercury per cubic millimeter of the volume of the discharge space, and in which a pair of electrodes opposite one another is connected to a feed device (Ex). The feed devices is used to start the above described discharge lamp and to supply the discharge current to the above described electrodes. The above described feed device (Ex) has the function of identifying that the discharge state of the above described discharge lamp is a glow discharge state, and that during the identification of the above described glow discharge state control is exercised such that the electric power supplied to the above described discharge lamp is set at less than or equal to twice as high as the nominal electric power of the above described discharge lamp.
In accordance with another aspect of the invention, the object is achieved in a light source device in which a discharge lamp which contains greater than or equal to 0.15 mg mercury per cubic millimeter of the volume of the discharge space, and in which a pair of electrodes opposite one another is connected to a feed device (Ex). The feed device is used to start the above described discharge lamp and to supply the discharge current to the above described electrodes. The above described feed device (Ex) consists of a ballast circuit (Bx) and a dc source (Mx) which is used to supply the above described ballast circuit (Bx) and that the above described dc source (Mx) exercises control with respect to its power supply capacity such that the power supplied to the above described discharge lamp is set at less than or equal to twice as high as the nominal electric power of the above described discharge lamp.
The invention is further described below with reference to the embodiments shown in the drawings.